Research

Kaito Yoda and Ai Yamakage, "Superconducting gap structures in wallpaper fermion systems", arXiv:2509.25823.
[Abstruct]
We theoretically investigate the superconducting gap structures in wallpaper fermions, which are surface states of topological nonsymmorphic crystalline insulators, based on a two-dimensional effective model. A symmetry analysis identifies six types of momentum-independent pair potentials. One hosts a point node, two host line nodes, and the remaining three are fully gapped. By classifying the Bogoliubov–de Gennes Hamiltonian in the zero-dimensional symmetry class, we show that the point and line nodes are protected by ℤ2 topological invariants. In addition, for the twofold-rotation-odd pair potential, nodes appear on the glide-invariant line and are protected by crystalline symmetries, as clarified by the Mackey–Bradley theorem.